1. Field of the Invention
This invention relates to optical fibres and to optical fibre gratings.
2. Description of the Prior Art
Optical fibre gratings are periodic refractive index modulation written either in the cladding of an optical fibre (the low refractive index region to confine light) or the core of an optical fibre (the high refractive index region to guide light) or both. In order to write the grating, a suitable fibre usually has a photosensitive core, a photosensitive cladding or both. A glass is photosensitive when its refractive index can be modified (usually, for these purposes, permanently) by optical radiation.
In silica based optical fibres, a core can be made photosensitive simply by incorporating germanium, which also has the desired effect of raising the core refractive index to form a waveguide. A high germanium content is usually used to achieve a large photosensitivity. This, however, gives the fibre a large numerical aperture which can lead to a large splice loss when connected to a standard telecommunication optical fibre due to its incompatibility with standard telecommunication optical fibres.
Numerical aperture (NA) in an optical fibre is a measure of acceptance angle of light which can be guided along the core and is defined as: ##EQU1## where n.sub.co and n.sub.cl are the refractive index of the core and cladding respectively.
The silica based cladding is normally transparent to the writing beams, giving easy access to the core from the side of the fibre.
It has been found that adding boron into a germanium-doped silica region can obtain an enhanced photosensitivity [reference 1 listed below]. Since boron incorporation into silica will lower the refractive index of the glass, any desired refractive index can be achieved by incorporating appropriate amount of boron and germanium. A strongly photosensitive fibre can thus be made to be compatible with standard telecommunication optical fibre to achieve a reduced splice loss.
Another method to achieve a much improved photosensitivity is low temperature hydrogen loading technique where a pre-fabricated optical fibre is placed in a high pressure H.sub.2 cell at room temperature for a few days to a few weeks to allow hydrogen to diffuse into the core region of the fibre [2]. A grating is written in the fibre before the hydrogen diffuses out. It is believed that photo-induced reaction of hydrogen with the glass components at bright part of the fringe pattern of the writing beam is behind the formation of gratings in this case. Refractive index change as high as 0.01 has been demonstrated with this technique enabling very strong gratings to be written in the core region of germanium-doped core of optical fibres.
In previous examples of writing optical fibre gratings, where only the core of an optical fibre is photosensitive, one imprinting technique involves exposing the side of the fibre to two coherent interfering optical beams [3]. The grating's pitch can be adjusted conveniently by changing the angle between the two beams. The writing beams in this case can be optical radiation at a wavelength between 160 nm (nanometers) to 300 nm. Using a photosensitive fibre core provides a large spatial overlap between the refractive index modulation and the guided optical transmission mode, since a large part of the optical power propagates in the core. Such an optical fibre is easily fabricated with current optical fibre manufacturing technologies.
Since the writing optical beam do not have to go through a thick photosensitive region which usually attenuates the writing beam strongly, a uniform exposure across the photosensitive core can be achieved. Such uniform exposure is very important to achieve gratings with certain desired characteristics, as will be mentioned later.
Another previously proposed method for writing fibre gratings is a technique using a phase mask [4]. A phase mask is a silica plate with many parallel periodic grooves written on it, and an image of the periodic pattern is produced in the space behind the phase mask when an optical radiation is directed onto the phase mask. A photosensitive fibre can be placed behind the phase mask for gratings to be imprinted in the photosensitive region of the optical fibre.
A significant feature of a uniform fibre grating is that it only reflects light at a certain resonant wavelength (Bragg wavelength) characteristic of the grating pitch, fibre parameters and the transverse field distribution of guided light. As a narrow band device, it has many applications such as reflectors for fibre lasers (particularly for single frequency fibre lasers), as band-stop filters, as band-pass filters, or in fibre sensors.
Another important application of optical fibre Bragg gratings is that they can be used to build add/drop filter for use in wavelength-division-multiplexing (WDM) systems [5]. In a WDM system, light at several different wavelengths propagates along the same optical fibre. An add/drop filter can take light at a particular wavelength out of the optical fibre or add light at a particular wavelength onto the optical fibre. Such a device is very important for a WDM system to operate.
A grating can also be made to be chirped, where different wavelengths are reflected at different point of along the grating by varying either the grating pitch or the fibre parameter or both along the grating. Such chirped grating has a strong dispersion because the reflected light at different wavelengths will have different path lengths. Such a grating can be used to compensate the dispersion in an optical fibre link and can also be used for optical pulse shaping [6].
In an optical waveguide, usually formed by a high refractive index region surrounded by a region with low refractive index, the total guided optical field can be divided into many basic elements with different transverse field distributions. Such basic elements are called modes of the waveguide [7]. Each mode is numbered by the order of the mode which gives the number of maximums in each dimension. The precise field distribution of each mode depends also on the waveguide parameters, such as core radius and NA of an optical fibre. Each mode is made not to be able to be further divided into other modes by introducing an orthogonality principle. In an optical fibre and a two dimensional polar co-ordinate, the orthogonality is expressed as ##EQU2## where NOI stands for normalised overlap integral; .psi..sub.mn (r,.phi.) and .psi..sub.kl (r,.phi.) are the normalised optical field distributions of LP.sub.mn mode and LP.sub.kl mode respectively.
In an optical fibre, the core size can be made small enough so that it can only support the lowest order LP.sub.01 mode. Such fibre is called single mode optical fibre and optical fibre is thereafter referred to single mode optical fibre. It eliminates the problem that a temporal optical pulse is broadened due to that each mode take a different path and arrives at the other end of the optical fibre at a slightly different time as in a multimode optical fibre. An optical fibre is usually coated with a high refractive index polymer. If such polymer coating is stripped off as in the case when a fibre grating needs to be written in a part of the optical fibre, the glass part of the optical fibre forms a waveguide with the low refractive index air surrounding it. This is a large waveguide which can supports many modes, which are called cladding modes.
When a grating is written in such an optical fibre, the guided LP.sub.01 mode do not only couple into the backward propagating guided LP.sub.01 mode at the main Bragg wavelength but also into all the other cladding modes. Coupling into each mode will happen at a different wavelength and a series of notches will appear in the transmission spectrum at the short wavelength side of the main Bragg wavelength.
The resonant condition for the coupling to occur is ##EQU3## where .beta..sub.01 (.lambda.) is the propagation constant of the LP.sub.01 mode, .beta..sub.mn (.lambda.) is the propagation constant of LP.sub.mn mode, and .LAMBDA. is the grating pitch. Where .beta..sub.mn (.lambda.) is replaced by .beta..sub.01 (.lambda.), equation (3) gives the resonant condition for the main Bragg wavelength. The effective refractive index modulation of the grating for the coupling between the guided LP.sub.01 and LP.sub.mn modes serves as a good measure for the coupling strength, ##EQU4## where .DELTA.n(r,.phi.) is the refractive index modulation of the grating.
The cladding mode is eventually absorbed by the high refractive polymer coating when it propagates beyond the stripped part of the fibre. If another light propagates at the wavelengths of notches as it would do in a WDM system, the part of the light will be lost. Such loss can be severe if a strong grating is used. If a media with a refractive index equal to that of the cladding glass is placed around the coating-stripped part of the optical fibre, the discrete loss notches turn into a continuous broad band loss as this is equivalent to a waveguide with very large dimensions which would support a very large number of very closely spaced modes. In the extreme case where the dimensions of the waveguide becomes infinite, the discrete cladding modes turns to a broad continuous band. If a media with a higher refractive index than that of the cladding glass is placed around the coating-stripped part of the optical fibre, the cladding modes are no longer supported by the waveguides and become radiation modes.
One proposed method to counter this problem of loss at the short wavelength side of the main Bragg wavelength is based on suppression of the normalised refractive index change for the coupling of the guided LP.sub.01 made into cladding modes by having a uniform photosensitive region across the cross section of the optical fibre [8]. From the orthogonality of the modes in equation (2), NOI(.infin.)s between the LP.sub.01 mode and the cladding modes are zero.
If a grating with a uniform refractive index change over the whole cross-section (A.infin.) of an optical fibre is written, .DELTA.n(r,.phi.) in equation (4) will be constant over A.infin. and can therefore be taken out of the integration in equation (4). What is left in the integration will be NOI(.infin.) which is zero. In another words, the LP.sub.01 mode will not couple into any of the cladding modes because the effective refractive index change for the coupling is zero. Since the LP.sub.01 mode only has field distribution over the core and the part of the cladding immediately next to the core, it is usually sufficient to have only this part of the optical fibre photosensitive.
Although it is possible to introduce a photosensitive cladding around a photosensitive core in practice, it is, however, very difficult to make the same photosensitivity over both cladding and the core. Even if such a fibre can be made, writing a grating with a uniform refractive index change over the whole photosensitive area can be very difficult as the writing beam is strongly attenuated as it penetrates into the thick photosensitive region. Another proposed method is to use a high NA fibre [9].
The use of the high NA fibre increases the gap between the main grating wavelength and the next cladding mode coupling wavelength, so it leaves a useful operation band. However, such band is only about 7 nm wide in a high NA fibre (0.25) and thus is much less then what is desired in many applications.
It is an object of the invention to provide a photosensitive optical fibre which suppresses the coupling from the guided modes into cladding mode or radiation modes by an optical fibre grating.